Does altitude affect AFUE? or just the input/output BTUs?

I searched and searched and searched yesterday for an answer to the question "Does altitude affect AFUE?" but did not come up with any definitive answers. Hopefully this discussion will answer that question more easily for folks in the future.

Derating combustion appliances typically applies the equation of about 2% per 1000 ft above sea level (from most manufacturer's specifications). So here in Santa Fe (at 7000 ft ASL), the derate would be 14%. Applying this to a 100 MBTUH input capacity appliance, the "natural" derating would make the appliance's input capacity actually 86 MBTUH. In all of the documentation I could find I could not determine if this derating ever affected the advertised AFUE or SSE rating, only the actual input/output in direct relation to the AFUE.

So my first question is, after an appliance has been naturally derated for altitude, can it be assumed to still function at the manufacturer's AFUE?

And then my second question is, assuming the above is true, and from what I have read here (http://hvac-talk.com/vbb/attachment.php?attachmentid=219042&d=1..., see appendix E), it would seem that Steady State Efficiency (and consequently, AFUE) actually improves with altitude. I know, right? Does anyone know at which rate the SSE (or AFUE) might improve with each 1000 ft increment above sea level?

Replies to This Discussion

Hi Rod, I'm certainly not an expert on this topic, but wanted to let you know someone is following to see if an answer pops up. And yes I hear the crickets as well.

The paper in the link was a bit too long for me to read it all so I'll take your word for SEE and AFUE both showing some improvement with altitude. Even though the total BTUs being produced decreases, it seems logical that the combustion process can be adjusted for the lower air pressure. If a furnace were tuned up at sea level and then installed at 7,000 feet I would expect it to perform poorly. But re-tuned, it should be close to "as manufactured"" in performance. A little better or a little worse I can't guess.

Interesting. Since we have air time I'll ask you another altitude question. If you transport a window from sea level to 7,000' do you allow the pressure to equalize during the trip and then seal the vent?

In reading that paper, just jump right to the appendix E as that paper has an immense amount of information. I generally read the whole thing and it took me a good hour and a half to assimilate. Good info there though, if you've got time.

To answer your question (or rather, to pose yet another one), do windows need to equalize to the outside pressure? I think all that will happen is that the windows will bulge outwards a bit from the pressure difference, but I don't see how it would affect R-value or functionality any. Do you?

I'm not positive that I have an answer for this as well but I am interested in the topic and would guess that the answer lie in the ASHRAE Standard 103 Method of Testing for Annual Fuel Utilization Efficiency of Residential Central Furnaces and Boilers. I have not read this Standard and do not have it in my library of ASHRAE Standards but I suspect that there maybe some information in that. Anyone familiar with this Standard?

So.... seriously? No one can answer this question about altitude affecting AFUE? Sheesh! Alrighty then, seems I'm going to have to bust out a can of triple dog dare for all you HVAC experts to jump on. Or can it be that folks really don't know? Seems unlikely...

First, it is the wrong question. AFUE is a comparative consumer number whereby the layman is presumably assisted in choosing an appliance by its advertised efficiency.

If we are addressing condensing appliances, all others being a waste of time and money, then we assume sealed combustion, direct vent and a negative pressure gas valve. Unless the appliance has a Lamda control it will have to be tuned to the application i.e. available gas pressure and to your question the gas/air ratio.

The significance of combustion efficiency at altitudes above sea level are nothing compared to temperature differential and heat coefficient mentioned in Appendix E.

When designing systems in Sante Fe we rarely see a furnace but spec. many condensing boilers derated for output but enhanced by low operating temperatures giving us steady state combustion efficiency well into to upper 90's.

Well, I don't really think I asked the wrong question, "Does altitude affect AFUE?" Because that's what I wanted to know. That's the number I use in many calculations, to present to the layman (my customers) accordingly.

About 50% of the homes I audit in the Albuquerque/Santa Fe area have furnaces, not boilers. And of those furnaces, about 90% are 80% AFUE, non-condensing units, 40% of which run on propane. The diversity here runs the gamut. My question stems from often seeing 80% efficiency units (boilers or furnaces) without any evidence of altitude adjustments, whether in the form of orifice sizing, pressure regulating, or via Lamda controls. I just don't see any labels indicating so. So I have to assume that the unit has not been adjusted for altitude, thus my need to know if the efficiency is affected. Does that make sense?

I think that my question has been answered offline, with much more of my research coming to the conclusion that altitude does not "significantly" affect efficiency. And at this altitude (7000 ft), it's about a wash, so, no need to adjust the AFUE.

I don't think that de-rating atmospheric boilers for altitude should affect efficiency, either "annual" or instantaneous - just output. If anything the efficiency might increase some as the heat exchanger in the de-rated boiler will be larger than originally designed for the burner input. De-rating the burner automatically up-rates the heat exchanger.

We work with Viessmann equipment. A major feature of their condensing equipment is that there is no need to de-rate for altitude.

Dale, I appreciate your comments, they make perfect sense. It seemed to me when I was researching this in the past that I came to the same conclusion as you, that it's actually possible the efficiency improves with altitude. Not sure I actually understand that concept fully, but I don't have to know it all ;)

Afue is not a real number. Look at system Efficiency BTU in vs BTU out, Its more real. Look at gas sold in boston most of east big citys are at 1050 BTU's per cub foot Kansas City is about 950-1050 BTU's per cub foot Denver is about 780-820 BTU's cub ft. Most Nat gas piped now has been striped of most LP gas and sulfur has been taken out. I watched a test lab run LP gas and Nat gas fire places for Efficiency and had to store the 4 hour run in a big tank before starting so would test the BTU's of gas. They tested for 14 things - water sulfur high BTU gases. All that "extra" things add to making a change in AFUE and CO rate.

Correct me if I am wrong, but you're saying "AFUE" but really mean "actual efficiency". AFUE is measured under controlled conditions in a lab at or near sea level, and has little in common with the actual efficiency Mrs. Jones will see from her installation. (I think this is what Morgan was getting at when he said this was the wrong question earlier).

If actual efficiency is defined as temperature rise per number of BTUs supplied, the following thoughts come to mind:

(1) Peak efficiency requires complete combustion, which requires the correct amount of oxygen per unit of gas. Unless the unit is equipped with a lambda (oxygen) sensor in the exhaust, it must be dialed in for proper combustion according to the manufacturer.

(2) As there is less oxygen at altitude, the fan will need to spin faster to deliver more CFM (increases electrical consumption, however fan wattage is negligible when compared to gas BTU).

(3) As most altitude gas is derated, we also need more cu.ft. of gas than at sea level.

(4) The heat transfer rate from flame to heat exchanger and heat exchanger to water is the same at sea level and at altitude.

(5) As boilers are typically derated at altitude (unless unit is over fired), the heat exchanger cross sectional surface area is available to transfer less BTUs, which increases efficiency (this is the reason why boilers are more efficient at lower modulation rates).

(6) Many installations at altitude use antifreeze, which reduces fluid heat capacity and therefore requires adjustments on the hydronic side which may reduce system efficiency. Other than that pure water's heat capacity does not change with altitude.

(7) Assuming (1) has been met, and ignoring (4) and (6), the temperature rise per BTU can be expected to be a bit better at altitude due to (5), resulting in a possibility for a bit higher efficiency.

Now let's keep in mind that a poorly designed system that causes the boiler to short cycle or a condensing boiler to run at high temperature has a much greater (negative) effect on actual (system) efficiency that altitude and everywhere else.